Optical spectroscopy of the neutral–ionic phase transition in a charge-transfer complex: TTF and tetrahalo-p-benzoquinones

Far-infrared reflectivity spectra have been investigated for a charge-transfer complex of tetrathiafulvalene and 2-bromo-3,5,6-trichloro-p-benzoquinone, which undergoes a neutral–ionic (N–I) transition at Tc=68 K. With decrease in temperature toward Tc, the far-infrared reflectivity spectrum is gradually transformed into a Drude-like high reflectance band edging around 100 cm−1, and the optical conductivity shows an anomalous peak around 10 cm−1. The far-infrared anomaly is related to the gigantic dielectric response just above Tc, which are both discussed in terms of dynamics of N–I domain walls with fractional charge.